So a self-written poem is worth less than a purchased fortune cookie?

I agree with your sentiment, but the retort would be that even a few minutes of your time has more monetary value than that cookie.

The diamond cartels love this kind of thinking. It should be about value, not price.

With a diamond, you are not getting value (except in some rare circumstances involving unusual stones).

It is not a good thing for courtship gifts to have value; if they have too much value they are likely to be dishonestly accepted by someone who isn't interested in the giver but does want the object.

If you're buying people because you're in a sexist world, agreed. Should probably stop doing that though.